High ohmic resistance conductor



March 1, 1955 R. w. BUCHANAN ET AL 2,703,356

HIGH OHMIC RESISTANCE CONDUCTOR Filed Feb. 1. 1951 WWMW United States PatentO HIGH OHMIC RESISTANCE CONDUCTOR Ross W. Buchanan, Robert W. Smith, and Taine G. McDougal, Flint, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application February 1, 1951, Serial No. 208,930

12 Claims. (Cl. 201--75) The present invention relates to an improved high ohmic resistance cable for internal combustion engine ignition systems and to a process for producing such a cable and is a continuation-in-part of our application Serial No. 99,756, filed June 17, 1949. M

It is well known that the reception'of radio broadcasting as well as television and other short-wave transmissions can be interfered with to a serious extent by the high tension ignition systems of motor vehicles. This interference is due to the oscillatory nature of the spark which covers a wide range of frequencies although interference is greatest in the radio frequency range.

Although the average energy which the ignition system of a vehicle must deliver to the spark plugs for engine operation is rather small, being less than that required to operate a car radio, and because of its impulsive nature, the ignition spark may expend energy at an exceedingly high rate even though for avery short period of time. These peak energies may be as high as 100 kilowatts and will produce interference and noise over a considerable area. The disturbance created is familiar to every car owner but is especially serious in vehicles using high frequency communication equipment. In addition, as the number of television and frequency modulation receivers increase, particularly in the metropolitan areas, interference from automobile traific may severely handicap reception.

There are several ways in which this interference and noise can be suppressed but only two have gained wide acceptance in the aircraft and automotive vehicle industry. One of these is the complete shielding of the ignition system. This does not reduce the generated noise but simply prevents it from being radiated to the nearby radio or television receivers. The other method comprises the insertion of resistor suppressors into the high tension leads of the ignition system. This method suppresses the interference by quenching or otherwise modifying the oscillatory portion of the spark discharge.

Complete shielding of the ignition system is very effective when properly carried out and is unafiected by the most adverse operating conditions. However, nstead of limiting the intensity of the spark discharge it is actually ice the spark. Where resistors of this type are used much of the power normally wasted during the spark discharge in heating the spark electrodes will be dissipated in the resistor. Extensive tests have shown that a marked decrease in electrode wear can be obtained even with relatively low resistance values.

As the radio interference becomes more serious it is evident that a unitary resistor at a single location in the ignition circuit will not give satisfactory noise suppression. On some cars resistors located at each spark plug terminal will meet the requirements while on others it is found to be necessary to install resistors at the distributor end of the ignition coil or in the distributor rotor arm itself as well as in each of the leads to the spark plugs from the engine. Where resistors of this type are used the resistor must be contained in a protective housing of some kind to prevent accumulation of moisture, oil or dirt which might attack the resistor or promote a destructive flash-over. This type of construction is not entirely satisfactory as the housing for the resistor which is usually made of a plastic or ceramic material is much more costly than the resistor itself so that the number of suppressor resistors required for a modern multicylinder engine constitutes an item of considerable added cost to the car manufacturer or owner.

The disadvantages of lumped resistors of this type and the expensive nature of entirely shielded systems has stimulated considerable interest in the possibility of using a distributed resistance in the ignition circuit as, for example, a high resistance conductor in the ignition cable itself, While such high resistances can be obtained by using extremely fine metallic wire the structural weaknesses of such a conductor would make it undesirable from the standpoint of fabrication into a cable by conventional methods. In addition such a cable would be extremely fragile in service. As an alternative, considerable interest has been directed in the past few years to the development of a conducting rubber as a conductive element for an ignition system. In this connection it was considered that the use of such a conductive rubber would be suitable for the production of a cable construction free from air voids and would thus eliminate the destructive corona eifects resulting from such a condition. Such a cable has been found to be satisfactory for the attenuation of radio frequency oscillations. In practice, however, conductive rubber has not been entirely satisfactory because of the inability to provide a good bond between the rubber conductor and the rubber insulation necessary.

, conductors which might serve more satisfactorily.

increased due to the added capacity of the shielding. This is a disadvantage as it results in increased spark plug electrode wear and excessive strain on the ignition coil and its associated high tension leads. Moreover, complete shielding is expensive and requires the perfect bonding of all of the joints. This presents a problem of maintenance as well as manufacturing. While shieldingispractical and used extensively in aircraft applications it is not considered as a satisfactory solution for automotive vehicle interference problems.

The second method of noise suppression and'the one which is now more commonly used in the automotive industry is the incorporation of one or more resistors with resistances of several thousand ohms into the high tension leads of the ignition system. Such resistance suppressors give satisfactory freedom from interferenceover-a wide Their elfect1veness,-however, depends frequency range. w more upon the quality of design than on the physicalrsize. Because of this it is entirely possible that a well designed small resistor might be superior to a rnuch larger resistor the design of whichis'poorly chosen. Resistors'of 1 this type can be assembled into small protective housings or fitted directly into the spark plug-core insulator itself.

Where it is desired that a resistance be in the form of an elongated flexible cable adapted to use in automotive engine ignition systems for providing ignition for the fuel and suppressing noise it is considered that the characteristics of primary importance are adequate strength of the core to withstand processing and the severe mechanical conditions to which the cable is exposed under operating conditions and suitable electrical characteristics over prolonged periods of operation including a stable resistanc larly desirable as carriers for high resistance conductive coatings. Although such materials have been provided with conductive coatings heretofore they have been-considered as not entirely satisfactory because of their unstable electrical characteristics and the destructive flaking of-the conductive coating. We have found that where such materials are treated in a particular manner conduc tive coatings may be made to adhere to fibrous materials with unusual tenacity and the coated fibers have electrical characteristics which are not affected by conditions gen- .tion of theresistor' is'to suppress the oscillatory portion of r I erally found in automotive vehicle operation, particularly stretching and flexing or conditions of atmosphere causing the absorption of moisture by wicking.

The media and the process for making the same which we desire to protect herein are specified with particularity in the appended claims and are described in the following specification and drawing in which: Figure l is a view illustrating schematically the flow sheet of the process for making high ohmic resistance cable in accordance with the present invention; Figure 2 is an enlarged view of a segment of the braided core of the cable; Figure 3 is a view taken along the line 3-3 of Figure 2; Figure 4 is an enlarged view of a segment of the insulated cable; Figure 5 is a view taken along the line 55 of Figure 4 and Figure 6 is an enlarged View of one fiber of the core illustrated as having a center of solid nylon and a surrounding shell of conducting graphite.

In accordance with our invention an exceptional combination of strength, durability and stable electrical characteristics is obtained by treating a cord of braided fibrous material with a wetting agent, often referred to as a surface active material or agent, to promote the adherence of a conducting material to the individual strands or fibers and thereafter applying a conductive coating of a desired thickness to the individual strands or fibers to obtain a conductor with a resistance suitable for the suppression of noise in the ignition system of an automotive vehicle as well as capable of carrying a high voltage current essential to the ignition of the combustible gases in an engine.

More specifically, we prefer to start with a braided cord of synthetic or natural fibers of commercial grade. As a first step any sizing or braid lubricant which becomes aifixed thereto during the manufacture of the braid is removed. The treated cord is then dried. When the braided cord has dried sufiiciently it is submerged in a solution of any suitable commercial wetting agent to impregnate the cord and again dried to provide each of the fibers of the cord with a coating or layer of the surface active material.

In this condition the cord is ready for the application of a conductive coating to the fibers. Coating 0f the cord with a conductive layer is accomplished by submerging the cord in a colloidal suspension of electrically conducting particles and a suitable liquid carrier, preferably water, to which a wetting agent has been added. A wetting agent is a soluble material which in small quantities will materially reduce the surface tension of the liquid used and promote wetting. In this connection, it will be obvious that many conducting materials would be suitable such, as for example, finely divided carbonaceous materials, finely divided metal powders having suitable electrical characteristics, such as aluminum, copper, iron, etc., or metal oxides, sulfides, carbides, nitrides, borides, etc., which have suitable electrical conducting characteristics. We have found that a colloidal suspension of carbonaceous materials, such as lamp black, graphite, etc., are preferable for coating the fibers and that exceptional results are obtained with a colloidal suspension of graphite in water to which a small quantity of Wetting agent has been added. In this connection our experience has shown that coatings of graphite are preferable as the graphite is easily obtainable in the form of commercial products such as aquadag, a colloidal suspension of graphite in water and has properties of resistance and stability making it unusually desirable as a resistor material.

The coated cord is then passed through a die of less diameter than the cord to force the coating solution to penetrate into the interstices of the braided cord structure, and provide a uniform surface coating of the conducting material on each of the fibers. Thereafter the coated cord is dried and subsequently submerged for a period of time in boiling water, then dried again. In this condition the coated cord is ready for the application of a suitable insulating coating which is accomplished in any of the well-known ways for applying a rubber insulative coating to a conductor. Where a single coating of the conducting material is suflicient to provide the electrical characteristics desired we have found that it is desirable to remove any of the wetting agent which may adhere to the outer surface of the coating before applying an insulative coating. The boiling of the coated cord in water prior to application of an insulative coating insures against any wicking action due to increased capillarity at the surface of the cord fiber which would permit moisture entering between the insulating coating and the conductive core or between strands of the core itself. In this conection, it will be understood that the boiling in water subsequent to the coating of the fibers, first with a wetting agent and subsequently with a conductive layer, does not affect the intermediate layer of wetting agent as it becomes locked between the conductive coating and fibers, the conductive coating being formed of extremely fine particles of conducting material which form a dense coating impermeable to the solvent action of the boiling water.

As the electrical resistance of the coated cord is dependent upon the thickness of the conductive coating on each of the fibers it may be desirable to subject the cord to a plurality of dipping steps in the coating solution depending upon the resistance per unit length of cord desired. In this connection, the cord may be provided with any number of additional coatings depending upon the electrical characteristics desired. Where a single coating is found desirable, the coated cord is treated with boiling water to remove any wetting agent adhering to the outer surface layer, as hereinabove described, and thereafter covered with an insulating coating. Where more than one coating application is desired, it is unnecessary to remove any adhering wetting agent as the layer of graphite is again submerged in the colloidal suspension of conducting material in water containing a small amount of wetting agent after first having been thoroughly dried. Subsequent coatings of the conducting material may be obtained in the same manner without requiring the step of submerging in boiling water. Where several coatings of conductive material are applied in this manner, it is desirable to remove excess wetting agent after the last coating has been applied to prevent the wicking action which may result and which has been referred to hereinabove. Subsequent to each of the separate applications of the coating material to secure the proper electrical characteristics, the coated cord is again passed through a drawing die of less aperture diameter than the diameter of the cord to obtain a uniform coating of the fibers with conducting material.

Particularly good results are obtained by utilizing a braided cord of a synthetic fiber formed of a polyamide such as nylon which comprises the high molecular weight copolymers of adipic acid and hexamethylenedi-amine. The size of the braided cord will depend upon the physical properties desired in the conductor. We have found that a cable having a core of a commercial grade of nylon cord treated to provide the conduction of electrical current is particularly suitable as a cable in the ignition system of vehicles. The cord which we prefer has a diameter of .063 inch and is composed of four center strands surrounded by a shell of braided strands. Each of the center strands in turn is composed of three strands of twisted nylon fibers. It will, of course, be understood in this connection that other forms of cord structure having comparable strength characteristics would be equally satisfactory.

As a first step the cord is treated to remove any sizing or lubricant which may adhere to the cord as a result of the processing of the commercial grade of nylon. This is accomplished by soaking the nylon cord in chloroform. In the drawing there is illustrated a spool 2 of commercial grade nylon mounted adjacent a tank 4 containing chloroform with the cord passing from the Spool into the tank containing chloroform and over a plurality of alternately displaced rollers 6 submerged in the liquid. In this connection although one chloroform bath may be sufiicient to remove all of the sizing and lubricant we prefer a series of several tanks or receptacles such as 4 through which the cord is successively passed and have found that three to six chloroform baths are preferable. Upon removal of the cord from the series of chloroform baths it is passed through a drying oven 8 where the cord is subjected to a temperature of 300-325 degrees F. for a period of about 3 to 5 minutes while passing through the oven. Upon being thoroughly dried in the oven 8 the cord is passed over a plurality of alternately displaced rolls 10 submerged in a solution of a commercial liquid wetting agent in water in a tank 12. The cord is subjected to this treatment to provide for improving the wetability and capillarity of the surface of the strands of the fiber for receiving the conductive coating.

The commercial wetting agents which we have found suitable in our process are those of the classes commonly known commercially as Santomerse and Aerosol and particularly Santomerse No. l and Aerosol AS of those classes. Santomerse No. 1 is an alkyl aryl sulfonate and Aerosol AS is an iso propyl naphthalenic sodium sulfonate. In treating the cord we prefer a solution of about 2% of one of the heretofore mentioned commercial compounds in water but have found that desirable results may be obtained with solutions containing from 25% to 5% of one of the aforementioned wetting agents with the balance water. After treatment in the wetting agent solution the cord is passed through a second drying oven 14 wherein it is dried at a temperature of about 300-325 F. for a period of about 3 to 5 minutes while passing through the oven. In this condition the cord is suitable for the application of a conductive coating.

To provide the cord with suitable electrical characteristics for use in a vehicle ignition system an electrically conductive surface is formed on the individual fibers of the braided cord. This is accomplished by passing the cord over a plurality of alternately displaced rolls 16 submerged in a solution of a colloidal suspension of graphite in water commonly known as aquadag and 2% of one of the aforementioned wetting agents contained in a tank 18. The aquadag has a graphite content of about 12% in water. In passing the cord through the solution in tank 18 the solution of aquadag and wetting agent is absorbed in the cord structure due to the previous treatment with the wetting agent forming a substantially solid coating on all of the individual fiber surfaces in the cord. However, to insure that the innermost interstices of the braided cord structure are penetrated and that the coating is of unform thickness on each fiber the cord is thereafter passed through a die 20 of substantially less diameter than the cord thereby forcing the liquid solution into the recesses formed by the braided structure Where it comes into contact with and coats the innermost fiber surfaces in the braided cord and simultaneously the squeezing action of the die provides a coating of uniform thickness on each fiber. With a braided cord of substantially .063 inch in diameter we have found that a die of substantially .055 inch aperture diameter is preferred for forcing the solution of aquadag and wetting agent into the innermost recesses of the braided cord structure to attain a uniform coat on each of the fibers. Of course it will be understood that with a cord structure of less diameter a die of proportionately less aperture diameter will be used. After the treated cord has been passed through die 20 it is passed through another drying oven 22 wherein the coated cord is dried at a temperature of about 300325 F. for a period of 3 to 5 minutes while passing through the oven. After the braided cord has been treated with the aquadag solution and dried it is passed over a plurality of alternately displaced rolls 24 submerged in boiling water in a tank 26. Its passage through the tank 26 requires a period of time not less than five minutes during which the coated cord is subjected to the effect of the boiling water. In this connection we have found that it is particularly desirable to construct the tank and roll assembly so that the coated cord is submerged in the boiling water for about thirty minutes. Thereafter the cord is passed through a drying oven 28 wherein it is dried at a temperature of 300 to 325 F. for a period of about 2 to minutes while passing through the oven and then wound on a spool 30 preparatory to applying a suitable insulative coating to the surface of the cord. The insulative coating may be of rubber or any other suitable insulative material and may be applied by any of the commonly used methods for applying insulative coatings to conductors.

As the thickness of the carbon coating on the braided cord fibers determines the electrical characteristics of the conductor it may be desirable to apply a second or a third or even more coats of the solution of aquadag and wetting agent depending upon the electrical characteristics desired. Where it is desired to employ the conductor in an automotive ignition system to provide a path for a high voltage spark across the spark plug terminals and at the Same time provide sutficient resistance in the circuit to dissipate any resonant condition resulting from the spark discharge at the electrodes it is considered that a resistance of substantially 5,000 ohms per foot is par ticularly desirable. To obtain this resistance characteristic we have found that a single coating of the fibers of the braided nylon is not sufiicient. On the other hand more than two coatings of the aquadag results in a resistance substantially lower than the desired 5,000 ohms per foot. A second coat of aquadag may be applied in the same manner as the first coat by including in succession in the apparatus train between the drying oven 22 and boiling tank 26 a tank similar to 18 containing a solution of aquadag and Wetting agent, a drawing die similar to die 20 and a drying oven similar to oven 22. Likewise a third coating may be applied by including an additional group of coating apparatus in the train. Each coating of conductive material is applied in the same manner as has been described hereinabove in connection with the first coat. The first coat of aquadag being dried, the coated cord is passed through the solution of aquadag and a wetting agent in the same proportions as hereinbefore described. It is then drawn through a die which, as in the previously described drawing step with .063 inch diameter cord, is of .055 inch in diameter. The second coat is then dried. If at this stage, the additional coating applied to the braided cord provides a .desired resistance characteristic, the coated cord may be prepared for the insulating coating in the same manner as described in connection with a single coating. If other coatings are desired, the step of boiling in water may be eliminated at this stage. Instead the cord is again passed through the coating solution and thereafter treated with boiling water before applying the insulating coating. With the conductive coatings applied in a thickness to obtain a. resistance of approximately 5,000 ohms per foot, a rubber insulative coating is applied to the coated cord and the finished cable is satisfactory for use in the ignition system of an automotive vehicle.

Referring now to Figures 2, 3, 4, 5 of the drawing there is illustrated an ignition-conductor made in accordance with the above described process. The conductor comprises a core 32 composed of four center strands 34 surrounded by a shell 36 of braided strands as hereinbefore described. Each center strand 34 is composed of three strands of a plurality of twisted fibers and each braided strand is composed of a plurality of twisted fibers. After treatment in accordance with our process each fiber is composed of a core 40 of nylon and a surrounding shell or coating 42 of graphite or uniform thickness throughout the length of the fiber with an intermediate layer of wetting agent. Further the graphite coating adheres to the nylon fibers without any destructive flaking due to the flexing or stretching of the cable. The core 32 is then provided with an insulative coating 38 of rubber or other suitable material which is bonded to the surface of the core. While we have described our process and conductor in connection with a conductive core of nylon it will be understood that core structures of other sizes, forms and materials such as glass fibers, cotton, rayon and linen may provide equally satisfactory results.

A conductor made in accordance with the process of our invention is particularly suitable for use in the ignition systems of vehicle because of its high tensile strength and stable electrical properties. Its electrical properties are unaffected by stretching, flexing or operation in at mospheres of high water vapor concentration where it might be expected that a wicking action would take place due to the fiber core which would vary the resistance. A conductor made in accordance with the process of our invention has an outstanding resistance to the destructive effects of corona discharge to the insulation.

What we claim and desire to secure by Letters Patent of the United States is:

l. A method of making an electrical conductor in cluding the steps of impregnating a non-conducting body of fibrous material with a liquid suspension of finely divided solid conducting material containing a wetting agent, drying said body, immersing said body in water heated to substantially its boiling point and again drying said body.

2. A method of making an electrical conductor including the steps of impregnating a non-conducting body of fibrous material with a wetting agent, drying said body, impregnating said body with a liquid suspension of solid conducting material, again drying said body, immersing said body in water heated to substantially its boiling point and again drying said body.

3. A method of making an electrical conductor including the steps of impregnating a non-conducting body of fibrous material in a liquid suspension of solid conduct ing material containing a wetting agent, drying said body,

again impregnating said body in a liquid suspension of solid conducting material containing a wetting agent,

again drying said body, immersing said body in water heated to substantially its boiling point, again drying said lgoccily, and thereafter applying an insulative coating to said 4. A method of making a conductor including the steps of impregnating a non-conducting body of fibrous mate rial with a water solution of a wetting agent, drying said body, impregnating said body with a suspension of a solid conducting material in water containing a wetting agent having substantially the same Wetting properties and chemically similar to said first-mentioned wetting agent, again drying said body, immersing said body in water heated to substantially its boiling point and again drying said body.

5. The method of making a resistance conductor from fibers of non-conducting organic material which comprises forming an elongated supporting structure composed of juxtaposed center strands of said fibers and a shell of braided strands surrounding said center strands, treating said structure with an organic solvent to remove processing lubricant and sizing from said structure, drying said structure, soaking said structure in a solution containing a wetting agent to improve the surface wetability of the fibers of said structure, again drying said structure, impregnating said structure with a suspension of graphite in water, again drying said structure, again impregnating the structure with said suspension, again drying the structure, immersing said structure in water heated to substantially its boiling point and again drying said structure.

6. The method of making a resistance conductor from synthetic fibers of a copolymer of adipic acid and hexamethylene-di-amine formed into an elongated supporting structure composed of a plurality of center strands of said fibers and a surrounding shell of braided strands which comprises the steps of successively soaking said structure in a solution of organic solvent to extract processing lubricant and sizing, drying said structure, dipping said structure in an aqueous solution of sulfonate wetting agent, again drying said structure, impregnating said structure with an aqueous suspension containing a sulfonate wetting agent and graphite, drawing said impregnated structure through a die having an aperture diameter less than the diameter of said supporting structure, again drying said structure, immersing said structure in water heated to substantially its boiling point, again drying said structure and thereafter applying an insulative coating of rubber to said structure.

7. A resistance conductor made by a process including the steps of impregnating a non-conducting body of fibrous material with a liquid suspension of finely divided solid conducting material containing a wetting agent, drying said body, immersing said body in Water heated to substantially its boiling point and again drying said body.

8. A resistance conductor made by a process including the steps of impregnating a non-conducting body of fibrous material with a wetting agent, drying said body, impregnating said body with a liquid suspension of solid conducting material, again drying said body, immersing said body in water heated to substantially its boiling point and again drying said body.

9. A resistance conductor made by a process including the steps of impregnating a non-conducting body of fibrous material in a liquid suspension of solid conducting material containing a Wetting agent, drying said body, immersing said body in water heated to substantially its boiling point, again drying said body and thereafter applying an insulative coating to said body.

10. A resistance conductor made by a process including the steps of impregnating a non-conducting body of fibrous material with a water solution of a wetting agent, drying said body, impregnating said body with a suspension of a solid conducting material in water containing a wetting agent having substantially the same wetting properties and chemically similar to said first-mentioned wetting agent, again drying said body, immersing said body in water heated to substantially its boiling point, again drying said body and thereafter applying an insulative coating to said body.

11. A resistance conductor made from a process including the steps of forming an elongated supporting structure composed of juxtaposed center strands of non-conducting fibers and a shell of braided strands of non-conducting fibers surrounding said center strands, treating said structure with an organic solvent to remove processing lubricant and sizing from said structure, drying said structure, soaking said structure in a solution containing a wetting agent to improve the surface Wetability of the fibers of said structure, again drying said structure, impregnating said structure with a suspension of graphite in Water to form a continuously uniform bonded coating of graphite on each of the fibers of said structure, again drying said structure, immersing said structure in water heated to substantially its boiling point and again drying said structure.

12. A resistance conductor containing synthetic fibers of a copolymer of adipic acid and hexamethylene-di-amine formed into an elongated supporting structure composed of a plurality of center strands of said fibers and a surrounding shell of braided strands, said conductor being made by a process comprising the steps of successively soaking said structure in a solution of organic solvent to extract processing lubricant and sizing, drying said structure, dipping said structure in an aqueous solution of sulfonate Wetting agent, again drying said structure, impregnating said structure with an aqueous suspension containing a sulfonate wetting agent and graphite, drawing said impregnated structure through a die having an aperture diameter less than the diameter of said supporting structure, again drying said structure, immersing said structure in water heated to substantially its boiling point, again drying said structure and thereafter applying an insulative coating of rubber to said structure.

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